Production of liquid hydrocarbons



April 1954 R. w. JOHNSTON, JR 2,674,319

PRODUCTION OF LIQUID HYDROCARBONS Filed Feb. 20 1951 INVENTOR. A 04AND W JaH/wro/guk ATTORJVEY Patented Apr. 6, 1954 UNITED STATES PATENT "OFFIGE PRODUCTION DF. LIQUID HYDRO'CARBONS :RowlandW.;J0hnst0n, J12, Bellaire, Tex assignor to The Texas Company, New Y0rk,.N.:.Y,,,a corporation of Delaware Application'Fbruary 20, 1951, Serial No. 211:965

(Cl. MiG-21) 11 Claims.

l Thepresent'invention relates to thep'roduction ofli'quid hydrocarbons from a producing well extending into aporousproducin'g formation cont'aining liquidhy'drocarbons and capillary water.

The present invention particularly contemplates increasing the rate of hydrocarbon production fromsucha'well and materially restricting the production of water. "Broadly, this is'accom- 'plish'e'd by substituting forconnate capillary water in the formation asolution having asubstantially'greater density than the density'of the normal formation -water. advantageously, the'solution of increased density" permeatesthe formation outwardly about'the'well bore and a'substantial vertical distance below the point at which oil flows into thewell'bore from the formation.

'The solution is necessarily one "containing a salt of high density and-substantialsolubiiityun water which does notcorr'espon'dingly increase the volume thereof. Advantageously it is 'insoluble in the hydrocarbon pha'se'astheresult of which itrehi'ains in theforma'tion in 'spite'of the flow of oil thru the formation into the well.

By substituting'this high clensitywetting. phase for the formation water below the point of 'hydrothe well free from the influence of corrosiveground waters. It also" materially increases the overallflow of oil.

As above intimated, this sheet follows from'the increase in pressure'difierential betweenthe nonwetting oil phase, and. the w'etting'water phase at the point of oil production. 5 In such .a formation the wetting phase'rises' through the small capillary interstices by capillary" action "from a water table surface somewhere below the point of Withdrawal. The hydrocarbon ci1,.or.n0nwetting phase occupies the remainder 'ofthe pore space, that is, the larger pore openings 01" interstices or the formation not filled bythe' capillary water. At any elevation. corresponding tofthe point bf 'produic'ti'on'in a. formation such asthe 2 above, the capillary-wetting:phase and the nonwetting oil--phase-areseparated by anon-water interface.

At the oil-water interface,-= the pressure inthe capillary wetting phase is lower than thepressure at the water tableby-an amount equal to the headofcapillary waten between it and-the water table. Likewise; at this interface; the pressure i the non-wetting or oil phase is-equalto the pressure or" theoil atthe izwater table, less the head'of oil above the water table.

In-each casegthe head of liquid is a function of the density of theliquidand the height of-the point of reference above the water table. However, since the density of the-oil'is-less-than the density of -the wetting or'water phase, the head of oil aloovethewaten phase is always less than that of the water for any-given elevation. "Moreover, at an interface located atany suchpoint of reference, the pressure of the oil is greater thanthe pressure of the water by the difference in relativeheads ofthe two fluids with reference tothe water table.

'Thisdifference is known as. thecapillary pressure and in typical producing formations usually amounts --to not more than 10 pounds persquare inch.

It is important to: note that if the pressure in the well is lower thanthe pressure of the oil in approducible formation then oil will manifestly flow out ottheformation. 1 If the well pressure is below not only the'pressure-of the oil,- but also that of the water, both-oil and water will be produced. But; if. the-well pressure'is above that or the water pressure" and below: that ot the .oil

pressure, oil exclusivelyzwill be produced.

In other" words, lay-adjusting the bottom. hole well pressure in a producing: well toav valueonly slightly exceeding that of the water pressure,

water production is eliminated. The differential between the pressure 'of-the oil as it exists in the producing formation and the pressure in the producing'well'opposite the formation :is known as thedrawdown. "Atsuch relatively low drawdown, however, the rate of oil flow from the formation is usually quite low.

In accordance with the present; invention sub- "stitution ofthe-connate; capillary wettingphase in the formation below the pointof reference; by

a wetting phase of substantiallyincreased density substantially"increases the head of-;the wetting phase above'thewater table without changing the head of oil ,above the-watertable. Therefore; "it, correspondingly"reduces-the water pressure"at"the' oil-water interface. Since the oil pressure remains the same, the differential between the oil and water at the interface, namely the capillary pressure, is substantially increased.

Therefore, the drawdown may be resumed at a substantially increased value with a materially increased flow of oil from the formation but without producing water. In other words, by increasing the capillary pressure the drawdown may be increased correspondingly without lowering the pressure in the well bore opposite the formation to a value approximating the water pressure, namely, the pressure at which Water commences to be discharged from the formation. The water accordingly, remains as the wetting phase within the most minute capillaries of the formation while the oil flows through the relatively larger pores into the well.

To more specifically illustrate the foregoing, reference is made to the attached drawing wherein Figure 1 is a schematic, sectional view of the lower portion of a producing well, and Figure 2 is a magnified, diagrammatic section through the formation adjacent the well bore taken along the plane 2-2 of Figure 1.

Actually, Figure 2 represents diagrammatically, in accordance with the convention which has been adopted in the art, a highly magnified section through the oil-bearing formation, showing the compacted grains I6 of the formation and the intervening porous interstices containing the oil and connate water phases. The formation, therefore, must be visualized as consisting of grains packed together as indicated, and Figure 2 exemplifies this arrangeemnt whether in a horizontal or vertical plane.

Referring to Figure 1, the numeral l8 represents a well bore extending downwardly and terminating at its bottom l2 within a porous producing formation Id. The line It: represents the surface of the water table at some point substantially below bottom l2 of the well bore or hole [0. However, since the subsurface brine in the water table Wets the formation surfaces, the resulting capillary attraction lifts the water above the water table through the more minute capillary interstices of the porous formation.

The water held in the formation by virtue of capillary attraction occupies the more restricted capillary passages or channels l8 of the internal pores, whereas the oil, being non-wetting with respect to the water-wet surfaces of the formation material, occupies the central or larger portion of the interstices as at 20.

As above indicated, at any given elevation (it) above the water table [5, as indicated by the level 2-2 in Figure 1, the capillary pressure (Cp) is, by definition, the difference between the pressure of the oil (P) and the pressure of the water (Pw) at oil-water interface 22. In other words,

Actually, it is the capillary pressure that lifts and holds the capillary water in the formation above the water table [5.

At the surface of the water table [5, however, the capillary pressure is zero, and the pressure of the oil and of the water (Pm) at this point are therefore identical.

At any elevation (h) above the surface 15 of the water table, the actual pressure in the water is less than the pressure at the water table (Pr), by an amount equal to the head of water in the capillary column. Actually, the head of water in this column is equal to its elevation (71 multi- 4 plied by its density (dw) terface 22:

Therefore, at the in- Po=Px-hdo Substituting these in the first equation:

Cp (capillary pressure) =Pa:hdw (Px-hdo) h(dodw) Therefore the difference in pressure between the oil and water at the interface 22 is dependent upon the distance (h) above the water table and the relative densities of the formation oil and the formation water; that is, due to its lower density, the oil has a higher pressure than the capillary water at the interface 22, and the difference is the capillary pressure (010) at the level 2-2.

In order to cause a liquid to flow from such a formation into the well bore, the pressure in the well bore must obviously be reduced below the pressure of the fluids as they exist in the formation. Therefore, if the pressure in the well bore at the level 2-2 is decreased below the oil pressure (Po) then oil will flow into the well bore l0. Likewise Water will flow into the well bore if the well bore pressure at level 2-2 is less than the water pressure (Pw) in the formation. But if the said well bore pressure is lower than Po but greater than Pw, only oil will be produced because obviously a liquid will not flow toward a region of higher pressure. The range of pressures at which oil will be produced to the exclusion of Water is the range between Po and Pw, and the breadth of this pressure range is equal to the capillary pressure (Cp) In accordance with the present invention, the capillary pressure (Cp) and accordingly the pressure differential, is increased by injecting a high density, aqueous solution from the well bore I0 into the producing formation. The solution, being compatible only with the formation water, freely enters the water-containing passageways or capillaries I8, and drains downwardly toward the water table l5.

As previously disclosed, the capillary pressure (Op) at any given elevation is dependent upon the relative densities of the oil and formation water. Therefore, since the injected solution has a materially greater density than the water already in the formation, then the capillary pressure at the interface 22 is materially increased. In other words, because the column of solution now held by capillary attraction in the formation between the level 2-2 and the water table is of greatly increased density, the pressure in the solution at the interface 22 is accordingly decreased, and since the oil pressure at the interface 22 remains unchanged, the difference between the pressure of the oil and the pressure of the solution at the interface 22, to Wit, the capillary pressure, is increased.

Accordingly, the pressure in the well bore 10 at the level 2-2 may be decreased far beyond that which would previously be possible without encountering a pressure sufficiently low to cause the water solution to flow toward the well bore. As a result of this increased pressure differential the rate of waterdree oil production is correspondingly increased.

The extent of the increase in capillary pressure thus realizable may vary, for example, from 30 to 300% depending on the density of the substituted wetting phase and the extent to which it is substituted for the connate, capillary formation water vertically below the point of oil production.

As a suitable substitute wetting phase, may be mentioned an aqueous solution of zinc chloride in sufficient concentration to form a wetting fluid miscible with but of substantially higher density than the connate water of the formation.

Saturation of the formation below the point of production with such a solution results in a corresponding increase in the drawdown pressure which can be applied without producing water. For example, a permissible non-water producing drawdown of pounds per square inch may be increased to about pounds per square inch by such treatment, with a corresponding increase in oil flow. A somewhat less, though substantial increase in non-water producing oil flow is, for the same reason, available where only a portion of the vertical extent of the formation below the point of withdrawal is thus treated.

In place of zinc chloride solution, above mentioned, there may be used solutions of magnesium chloride, potassium carbonate, sodium bromide,

stannic chloride, and any other soluble salt which forms a relatively dense solution miscible with the formation water, and which is compatible with and wets the formation.

A 69.4% zinc chloride water solution has a density of 1.93 gm./cm Bearing in mind that the capillary pressure at any elevation is dependent on the difference in density between the two phases, then the following is illustrative:

GM./cm. /cm. Typical density of formation water 1.10 Typical density of formation oil 0.73 Normal capillary pressure gradient .37 Density of zinc chloride solution 1.93 Density of formation oil .73

Capillary pressure gradient of treated formation 1.20

These data show that the capillary pressure gradient is altered by more than 300% merely by substituting for the capillary formation water a salt solution which has a density only about seventy-five per cent greater than the formation water. At any point above the water table the relative capillary pressure is correspondingly affected and at points substantially thereabove the actual increase in capillary pressure may be substantial. Thus, as above indicated, a normal capillary pressure of about 10 p. s. i. in the formation adjacent the point of oil withdrawal into the well may by this treatment be increased so that a drawdown of close to 30 pounds per square inch may be imposed without producing water.

As thus indicated it is advantageous not only to use salts which tend to substantially increase the density of their aqueous solutions, but particularly to employ relatively concentrated solutions of salts which have a substantial solubility in water.

In general it is contemplated employing solutions having a density at least 10% greater than that of the formation water although preferably, as above shown, solution densities at least 50% and preferably above 70% above the formation water are to be desired. Manifestly, there is no upper limit of density other than that imposed by the salts available.

As above intimated, impregnation of the formation is effected for substantial radial distance outwardly from, as well as, for a substantial vertical distance below the point where the oil is withdrawn into the well bore.

The extent of radial treatment is determined by the pressure gradient radially through the formation as a result of the drawdown imposed in the well. Thus, the formation must be treated radially outwardly from the well at least to the point where the decrease in pressure resulting from the intended well drawdown tends to be as great as the normal capillary pressure of the formation. Otherwise, as shown above, water will flow from the untreated area into the treated section. However, in a radial system such as the present, the pressure gradient sharply decreases in a radial direction outwardly from the bore as explained by Calhoun, Paper #315, Oil & Gas Journal, Darcys Law, Radial Flow System (issue of January 8, 1948, page 83) This necessarily means that the pressure differential imposed on the formation by a given drawdown at the well bore sharply decreases in the region close to the well bore.

It is for this reason that relatively limited radial treatment of the formation is normally sufficient to insure substantial freedom from water production at substantially increased rates of oil flow. Treatment for a distance of about 20 radial feet about the well bore is usually adequate. As will be apparent from the foregoing, however, the extent of radial treatment may be greater or less than this value, depending upon the actual increase in capillary pressure realized by the treatment. Thus the minimum radial treatment should increase in accordance with the actual magnitude of increase in capillary pressure. Moreover, to positively assure against even remote possibility of drawing water from some relatively distant point in the formation, some excess of treatment is desirable.

For example, when the capillary pressure is increased by as much as 20 pounds per square inch over its normal value at the point of production, and the newly imposed drawdown is to be increased by 20 pounds per square inch, then it is preferred to treat the formation for a distance of about 50 feet radially outwardly about the well bore. Where the increase in capillary pressure amounts to about 10 pounds per square inch and the drawdown is to be correspondinglyincreased, treatment for 25 radial feet is ample. In any case treatment of the formation for at least 10 radial feet is contemplated.

The vertical impregnation of the formation is preferably advantageously effected by a step" wise introduction of the salt solution in which the required radial saturation is effected and the well then shut in for a period of time sufiicient to permit excess solution to displace the underlying capillarywater. These steps may be repeated a plurality of times to insure the maximum vertical permeation.

They are rendered effective by the fact that the wetting phase saturation of the formation tends at all times to adjust itself to equilibrium conditions. Therefore, when the area about the well bore is saturated with the solution the excess proceeds to drain through the formation capillaries moving the normal formation water ahead of it. Accordingly, repetition of the saturation and draining steps ultimately effects dis;-

aemgam I placement of the formation water down;to. the water table.

According to one illustrative example, a producing well extends into an oil bearing formation containing a capillary formation water having a capillary pressure about the well bore of approximately 9 pounds per square inch. Any drawdown on this well in excess of 9 pounds per square inch results in the production of water in amounts increasing as the drawdown is increased above 9 pounds per square inch. At a drawdown of 8 pounds per square inch there is no appreciable water production but the rate of oil production is only 500 gallons per day. The well is treated in accordance with the present invention by injecting a concentrated aqueous solution of zinc chloride. Measured injection of the zinc chloride solution is continued in a volume approximately sufficient to saturate the formation for a distance about 50 feet radially from the point where the well bore meets the producing formation. The injection is then terminated and the well closed in for a period of about one hour.

After this time zinc chloride solution is again injected in approximately the same amount as before and the well is again closed in for about one hour. This treatment is repeated four times, sufficient to displace the capillary formation water by the zinc chloride solution in an amount in excess of that sufficient to reach the water table. Thereafter, by means of the actuation of a bottom hole pump, a drawdown of about 24 pounds per square inch is imposed.

The oil flow proceeds from the formation free from any appreciable amount of water and continues at the rate of about 1500 gallons per day.

It is apparent from the foregoing that the rate of water-free oil production is quite materially increased in the well in question. This is manifestly of considerable importance from the standpoint of eliminating the substantial pumping cost of the produced water, the obviation of well corrosion problems and the advantageous effect on formation pressure of the retained formation water.

While one method of effecting vertical impregnation of the formation has been given above for purposes of illustration, it may be understood that the present invention is not limited to this specific procedure but contemplates saturating a substantial vertical portion of the formation below the point of production by any effective means.

So also the present treatment may be combined with other effective Well treatments. For example, it is contemplated associating within the salt solution any compatible wetting or surface active agent capable of lowering the interfacial tension between the non-wetting oil and the aqueous wetting phase and thereby reducing the water saturation of the formation. Actually such a combination is particularly advantageous as regards displacement of the formation water by the salt solution since the tendency towards desaturation caused by the interfacial tension reducing agent materially facilitates the downward penetration of the salt solution. Alternately, instead of combining the interfacial tension reducing agent with the salt solution, a brief treatment of the formation with such an agent may precede, and also succeed, injection of the salt solution. Examples of such agents are polyoxyethylene sorbitan monolaurate, polyoxysubstantially ethylene sorbitan' monopalmitate andpol'yoxy ethylene lauryl alcohol.

It is particularly important to note that the displacing solution tends to remain in the formation for long, indefinite periods of time provided the drawdown pressure is not permitted materially to exceedIthe resulting increased capillary pressure in the adjacent sections of the formation. This follows from the absence of any'tendency for flow in the wetting phase, discussed above. Therefore, the solution is not withdrawn from the formation in the oil flow but tends to remainwithin the minute capillary passages in which it is disposed by the treatment. After a substantial period of time the improved effect may progressively tend to diminish but can be readily reimposed by simply repeating the foregoing treatment. Thus, the

wall may be kept regularly at a condition of substantially improved water free production.

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In the production of liquid hydrocarbons from a producing well extending into a porous producing formation containing a non-wetting oil phase and a wetting phase of capillary formation water a substantial distance above a water table wherein substantial quantities of water are produced with the oil at drawdowns greater than the capillary pressure in the formation adjacent the well bore, the improvement of substantially increasing water-free liquid hydrocarbon production which comprises injecting into the formation about the well bore and for a substantial vertical distance below the point of production from the formation, an aqueous salt solution having a density substantially greater than the density of the formation water, said salt being miscible with said formation water, insoluble in hydrocarbons and compatible with said formation, causing said injected solution to penetrate a substantial distance vertically below the point of production from said formation and displace therein the capillary formation water thereby substantially increasing the capillary pressure in the formation about the point of production, and thereafter producing liquid hydrocarbons from said well at a drawdown substantially above the original capillary pressure but not above theincreased capillary pressure without producing water.

2. In the production of liquid hydrocarbons from a producing well extending into a porous producing formation to a point substantially above a water table, said formation containing a non-wetting oil phase and a wetting phase of capillary water wherein water production commences at a relatively low drawdown exceeding the capillary pressure of the formation the improvement of substantially increasing water-free liquid hydrocarbon production by, injecting into the formation for substantial radial distance about the well bore an aqueous salt solution having a density substantially greater than the density of the connate formation water, said salt being substantially oil insoluble and. forming an aqueous wetting phase compatiblewith the for mation and miscible with the formation water; causing said injected solution topenetrate verti- I cally downwardly from the point of production in the well a substantial vertical distance toward the water table and displace therein the capillary formation water, thereby substantially decreasing the aqueous phase pressure and increasing capillary pressure at the point of oil production in the formation, and thereafter withdrawing liquid hydrocarbon from the formation into the well by maintaining in the well bore opposite the formation a pressure substantially below the initial aqueous phase pressure at a substantially increased rate of oil production under substantially non-water producing conditions.

3. In the production Of liquid hydrocarbons from a producing well extending into a porous producing formation above a water table and containing a non-wetting oil phase and a wetting phase of capillary formation water, wherein substantial quantities Of water are produced with the oil at drawdowns greater than the capillary pressure in the formation adjacent the well bore the improvement of substantially increasing water-free liquid hydrocarbon production which comprises injecting into the formation about the well bore, an oil-insoluble aqueous salt solution which is miscible with the formation water having a density substantially greater than the density of the formation water thereby displacing the capillary formation water, continuing said injection under a pressure and in an amount sufficient to displace said capillary formation water for a substantial radial distance about the well bore and for a substantial vertical distance below the point of production from the formation, thereby substantially increasing the capillary pressure in the formation about the point of production and thereafter producing liquid hydrocarbons from said well under substantially non-water producing conditions at a drawdown substantially higher than the original capillary pressure but not in excess of the increased capillary pressure of the formation at an increased rate.

4. The process according to claim 3 wherein said aqueous salt solution has a density at least 10% greater than that of the formation water.

5. The method according to claim 3 wherein said salt solution has a density of at least greater than that of the formation water.

6. The method ccording to claim 3 wherein said salt solution has a density of at least greater than that of the formation water.

'7. The process according to claim 4 wherein said solution is an aqueous solution of zinc chloride.

8. The method according to claim 4 wherein the formation is treated for a distance at least 10 radial feet outwardly from the well bore.

9. The method according to claim 4 wherein the formation is treated for a distance at least 25 radial feet outwardly from the well bore.

10. The method according to claim 4 wherein the formation is essentially saturated with said salt solution throughout substantially the entire vertical distance between the elevation of the well bore and that of the water table.

11. The method according to claim 4 including treatment of the formation with a surface active agent effective to substantially reduce the interfacial tension between oil and water.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,024,119 Vietti et al Dec. 10, 1935 2,246,725 Garrison June 24, 1941 2,402,433 Munn June 18, 1946 2,465,237 Larsen Mar. 22, 1949 2,490,291 Wrightsman Dec. 6, 1949 

1. IN THE PRODUCTION OF LIQUID HYDROCARBONS FROM A PRODUCING WELL EXTENDING INTO A POROUS PRODUCING FORMATION CONTAINING A NON-WETTING OIL PHASE AND A WETTING PHASE OF CAPILLARY FORMATION WATER A SUBSTANTIAL DISTANCE ABOVE A WATER TABLE WHEREIN SUBSTANTIAL QUANTITIES OF WATER ARE PRODUCED WITH THE OIL AT DRAWDOWNS GREATER THAN THE CAPILLARY PRESSURE IN THE FORMATION ADJACENT THE WELL BORE, THE IMPROVEMENT OF SUBSTANTIALLY INCREASING WATER-FREE LIQUID HYDROCARBON PRODUCTION WHICH COMPRISES INJECTING INTO THE FORMATION ABOUT THE WELL BORE AND FOR A SUBSTANTIAL VERTICAL DISTANCE BELOW THE POINT OF PRODUCTION FROM THE FORMATION ABOUT THE WELL BORE AND FOR SOLUTION HAVING A DENSITY SUBSTANTIALLY GREATER THAN THE DENSITY OF THE FORMATION WATER, SAID SALT BEING MISCIBLE, WITH SAID FORMATION WATER, SUBSTANTIALLY INSOLUBLE IN HYDROCARBONS AND COMPATIBLE WITH SAID FORMATION, CAUSING SAID INJECTED SOLUTION TO PENETRATE A SUBSTANTIAL DIS- 